Typically, plasma is a collection of charged particles containing approximately equal numbers of positive and negative charges and can be used in certain processing systems which are useful for a wide variety of applications. For example, plasma processing systems are of considerable use in the manufacture and processing of semiconductors and integrated circuits, both for etching and layer deposition on substrates, such as, for example, semiconductor wafers.
Generally, the basic components of such a system include a plasma chamber enclosing a processing region in which plasma is formed and a pumping region connecting to a vacuum port. Other basic components of such a system generally include a wafer supporting chuck, which is connected to a RF power supply in order to accelerate the plasma ions to strike the wafer surface with a desired energy. This RF power may often produce the process plasma; however, an additional electrode or RF antenna can be used to produce the process plasma.
The chuck is normally cylindrical and flat, supporting a 200 to 300 mm, or larger, diameter wafer for processing. For efficient use of chamber space, i.e. maximizing gas flow uniformity and minimizing the reactor footprint, process gases are injected above or around the plasma region, and the used gases are removed through an annular passage between the chuck and the sidewall to the vacuum pumping port provided in the lower portion of the vacuum chamber. With a large mass flow rate of processing gases, a large pumping speed for removing the used gases is a critical issue relating to process performances, such as the etch rate, high aspect ratio etch, feature profile, damage and contamination. The gas conductance of the annular region often severely restricts the pumping speed delivered to the processing region.
The possibility of employing plasma vacuum pumping in plasma processing systems has been described, for example, in U.S. Pat. No. 4,641,060, which issued to Dandl on Feb. 3, 1987. This system required “magnetized plasmas” and does not appear to be particularly suitable for typical plasma process systems.
Plasma vacuum pumps are capable of pumping a variety of gasses, including hydrogen and helium, with relatively high efficiencies, and are relatively immune to damage by solid or corrosive materials. The operation of such plasma vacuum pumps generally involves transforming a neutral gas into a plasma which then may be magnetized or magnetically compressed so as to be guided through suitable structures, such as a conduit. “Magnetized plasmas” as used herein is a plasma in which the electron flow is magnetized, i.e., the electrons circulate around the magnetic field lines. Momentum can be imparted to the plasma as a result of various electromagnetic interactions and can be imparted to the neutral gas through collisions between molecules of the neutral gas and moving ions which have been accelerated and have greater momentum than background gas. Therefore, the plasma can be pumped from the processing region to a second region, such as a discharge plasma region, which is generally maintained at a higher pressure than the processing region.
In the plasma vacuum pumping cell, described by Johnson et al. in pending U.S. patent application Ser. No. PCT/US99/12827, a plurality of magnets is positioned relative to the conduit in a manner to provide lines of magnetic force that extend along the conduit; and an electric potential source is disposed relative to the conduit to create an electric field which accelerates the ions from the conduit to the second region. A plurality of these pumping cells is arranged in a large area interfacing the plasma. This plasma vacuum pump utilizes the process plasma as the pumping medium, and, therefore, the pumping speed suffers for process systems that do not provide a high density plasma or dispose a sufficiently large pumping area interfacing the plasma.
Furthermore, the effluent gases from the process plasma usually include perfluorocompounds (PFCs), such as CF4, CHF3, C2F6, C3F8, C4F8, C5F8, SF6, and NF3, which are considered as greenhouse gases. In 1996 the American semiconductor industry signed a memorandum of understanding with the United States Environmental Protection Agency in which manufacturers committed to reducing emission of the greenhouse gases.
In the prior plasma pumps, the pumping action is severely limited by the plasma density at the input of the plasma pump. There is a need for increasing the plasma density at the input to the plasma pump to increase the pumping efficiency.